The present invention relates to a focus detector for use in optical systems, and more particularly, to a focus detector which detects the direction and magnitude of displacement of the focus of an optical system when out of focus, in response to relative changes in the positions between images produced by respective light beams which are obtained by splitting a pupil of the optical system when out of focus.
It is well recognized that images formed by respective light beams which are obtained by splitting a pupil of an optical system shift their relative positions in different directions. The principle of such a phenomenon will be described by way of an optical system shown in FIG. 1. In FIG. 1, which is a diagrammatic representation of an arrangement for detecting the focusing condition of an optical system, a blind 2 having an opening 2a is disposed adjacent to a pupil of a focusing lens 1 and a screen 3 is disposed in the rear of the lens 1.
If an in-focus condition is defined as the condition in which an image Q of an object (not shown) on the left of the lens 1 is formed on the screen 3 at Q', then blurred images Q.sub.1 and Q.sub.2, corresponding to a front focus and a rear focus condition, respectively, will be formed with respect to the image Q in positions shifted in opposite directions and along a direction perpendicular to an optical axis O when an out of focus condition exists. FIG. 1(b) shows the case where the opening 2a of blind 2 is shifted on the reverse side with respect to the optical axis O. While an image Q' is formed on the screen 3 when an in focus condition exists, blurred image Q.sub.1 ' and Q.sub.2 ' corresponding to a front focus and a rear focus, respectively are formed on the screen 3 when an out of focus condition exists. Thus, when the opening 2a is moved from the position shown in FIG. 1(a) to the position shown in FIG. 1(b), for example, the images Q and Q' lie as in the same position when in focus condition exists, resulting in no displacement of the image. However, when in a rear focus condition an image displaces from the position Q.sub.1 to Q.sub.1 ' and when in a front focus condition an image displaces from Q.sub.2 to the position Q.sub.2 '. Consequently, when the displacement of blurred images as described above is detected electrically by arranging a photoelectric detector at screen 3, it is possible to construct a focus detector.
By way of example, an apparatus which uses the foregoing principle to detect an in-focus condition is disclosed in the Japanese patent publication No. Sho 56-13929 in which a one dimensional solid-state image sensor is employed as a photoelectric detector for detecting image information provided on a screen. With such apparatus, an analog or digital operation such as correlation is essential to obtain relative positions of images. It is necessary, therefore, to amend the relative positions of images because a difference in a light amount due to eccentricity and the like of a pupil affects the accuracy with which a phase shift can be detected. In this case, while the displacement in the relative position of images can be detected only in one direction, it is impossible to detect displacement in the direction perpendicular to the one direction. By way of example, it is impracticable to detect an in-focus condition for images which have a change of light intensity only in the perpendicular direction. Therefore, phase shifts of two dimensional images can be detected employing a two dimensional image sensor and splitting a pupil in two dimensions. It should be noted, however, that the computational amount for the two dimensions is greatly increased compared with the one dimension with the resultant difficulty of conducting the computation (due to increase in the computational amount substantially in proportion to the square of the dimensional number of plane).